Turbulence Promoters Research Articles

Comparison of the Behavior of the FM01-LC Reactor with and without Turbulence Promoter Using CFD

Filter-press-type electrochemical reactors are the most common ones used in laboratory and industrial applications [1]. The FM01-LC is abundantly described in literature[2], where global and mass transport to electrodes [3-5], pressure drop and hydrodynamics [4] have been studied. However, these phenomena are still not fully understood.Despite the work realized about modelling and Characterization of the FM01-LC reactor through residence time distribution, direct flow visualization, axial dispersion models and CFD, not yet submitted a study where comparing experimental and simulated data of the FM01-LC reactor including from physical manner the turbulence promoter in the flow simulation. In this way is considered the study of the comparison of the hydrodynamic behavior in the FM01-LC reactor by residence time distribution (RTD) and fluid visualitation (FV), this comparison made by experimental work and CFD, with and without turbulence promoter for both cases.The flows studied were 0.5, 1, 1.5 and 2 Lpm, for both cases.The results obtained for the RTD and CFD for the flows studied and with and without turbulence promoter shown that the model used (κ-ε) can predict the experimental behavior, because the graphics obtained are very similar, both in the graphic without turbulence promoter as with turbulence promoter and the results obtained for the FV and CFD for flows studied (0.5 Lpm) and with and without turbulence promoter is shown that the fluid behavior is very similar for both cases at time of 2 seconds. The best distribution of the fluid is observed with the turbulence promoter.In conclusion the models used (κ-ε and convection diffusion) to predict the experimental behavior in the FM01-LC reactor functioned properly. It could also simulate the turbulence promoter from physical way obtaining similar values compared with the experimental data. The same way, in the FV the images have shown the same behavior of the flow inside of the reactor. In conclusion it can say that the κ-ε and the convection diffusion models can predict the behavior of the FM01-LC in the worked conditions. Besides using turbulence promoter a better fluid distribution is achieved.It is important to mention that the simulation made with the turbulence promoter with CFD is new because the promoter is drawing from physical way, so this result can be more exact that if used another models to simulate the space occupied by the turbulence promoter.References.1. Bengoa, C., et al., Characterization and Modeling of the Hydrodynamic Behavior in the Filter-Press-Type FM01-LC Electrochemical Cell by Direct Flow Visualization and Residence Time Distribution. Industrial & Engineering Chemistry Research, 2000. 39(7): p. 2199-2206.2. F.C. Walsh and D. Robinson, Electrochemical Filter-Press Reactors, 1998.3. Brown, C.J., et al., Studies of space-averaged mass transport in the FM01-LC laboratory electrolyser. Journal of Applied Electrochemistry, 1993. 23(1): p. 38-43.4. Griffiths, M., C.P. de León, and F.C. Walsh, Mass transport in the rectangular channel of a filter-press electrolyzer (the FM01-LC reactor). AIChE Journal, 2005. 51(2): p. 682-687.5. Brown, C.J., et al., Local mass transport effects in the FM01 laboratory electrolyser. Journal of Applied Electrochemistry, 1992. 22(7): p. 613-619.

Measurement of Local Convective Heat Transfer Coefficient of an Artificialroughened Duct by Using Liquid Crystal Thermography (LCT) Technique

The heat transfer from surfaces may in general be enhanced by increasing the heat transfer coefficient between a surface and its surroundings, and by increasing the heat transfer area of the surface, or by both. In most cases the heat transfer is concerned with the application of artificial surface roughness techniques as a turbulence promoter for convective heat transfer enhancement. To measure the transient and local convective heat transfer coefficient of an artificial roughened duct using liquid crystal thermography (LCT) technique, an experimental setup have been designed and developed. The aspect ratio of duct was maintained as three with relative roughness height ratio (e/D) of 0.13 and angle of attack is 900 for the range of Reynolds number from 7000 to 17500.

Investigation of pressure effects on the small scale wrinkling of turbulent premixed Bunsen flames

High pressure turbulent premixed flames have been significantly studied in the past decade. The eloquent flame images obtained under high pressure conditions attest the large changes in flame front wrinkling enhanced by the pressure increase. Among the explanation attempts, pressure was assumed to promote Darrieus–Landau (DL) instabilities. More recently the role of the laminar flame thickness was also identified as a key parameter for the flame/turbulence interactions. The objective of the current study is to contribute to this debate by isolating the effects of the suspected parameters. To do so, turbulent methane/air flames are investigated in a high pressure combustion test facility. A multi-grid turbulence promoter system has been developed and implemented to obtain a more intense, isotropic and homogeneous turbulence at the burner exit. The pressure range is 0.1–0.4MPa, and the mixture composition is varied between ER=0.7 and ER=1.0. Instantaneous flame images have been collected using Mie scattering tomography and exploited to analyze flame–turbulence interactions under controlled conditions. Flame and turbulence parameters have been independently varied under DL instability free conditions to isolate the effect of the laminar flame thickness and that of the small scale turbulence eddies. The stretching of the turbulence energy spectrum towards smaller turbulent length scales is identified as the main reason for the enhanced flame front wrinkling under high pressure flame conditions, together with the reduction of the laminar flame thickness. The Taylor micro-scale appears to be the regulating turbulence scale for flame–turbulence interactions.

Mass and heat transfer enhancement at the wall of cylindrical agitated vessel by turbulence promoters

Rates of liquid–solid mass transfer at a wall of stirred tank reactor lined with an array of vertical tubes turbulence promoter were studied using the dissolution of copper in acidified dichromate technique. Parameters studied are physical properties of the solution, impeller geometry, impeller rotation speed, cylinder diameter, circumferential distance between the vertical cylinders, and the effect of drag reducing polymers. The presence of vertical cylinders at the reactor wall increased the volumetric mass transfer coefficient compared to the presence of the wall without the cylinders by an amount ranging from 12.5% to 214.5%. The presence of Polyox WSR-301 drag reducing polymer decreased the rate of mass transfer by an amount ranging from 3.5% to 32.26% depending on polymer concentration and impeller rotation speed. All data were correlated by dimensionless mass transfer equations. The importance of the present results in building high space time yield catalytic reactors suitable for conducting diffusion controlled reactions was highlighted, also the possible role of the vertical tube array as a secondary built in heat transfer facility which assists external cooling jacket was noted.

Heat transfer enhancement of impinging jets with fractal-generated turbulence

A new passive method for the heat transfer enhancement of circular impinging jets is proposed and tested. The method is based on enhancing the mainstream turbulence of impinging jets using square fractal grids, i.e. a grid with a square pattern repeated at increasingly smaller scales. Fractal grids can generate much higher turbulence intensity than regular grids under the same inflow conditions and with similar blockage ratio, at the expense of a slightly larger pressure drop. An experimental investigation on the heat transfer enhancement achieved by impinging jets with fractal turbulence promoters is carried out. The heated-thin foil technique is implemented to measure the spatial distribution of the Nusselt number on the target plate. The heat transfer rates of impinging jets with a regular grid and a fractal grid insert are compared to that of a jet without any turbulator under the same condition of power input. A parametric study on the effect of the Reynolds number, the nozzle-to-plate distance and the position of the insert within the nozzle is carried out. The results show that a fractal turbulence promoter can provide a significant heat transfer enhancement for relatively small nozzle-to-plate separation (for a distance equal to 2 diameters an increase of 63% at the stagnation point, and 25% if averaged over an area of radius equal to 1 nozzle diameter, are respectively found with respect to the circular jet, against 9% and 6% for the regular grid in the same conditions of power input).

Hydrodynamics and mass transfer coefficients for a modified Raschig ring packed column

The pressure drop, the liquid holdup, as well as the liquid film mass transfer coefficients (kL) for a modified Raschig packing, with turbulence promoters, used in absorption columns, were determined experimentally. The aim of this work is to verify the improved mass transfer properties of this new packing for the randomly and, particularly, for the arranged packed columns. The experiments were performed at gas velocities ranging from 800 to 2,000 m h−1 and liquid velocities scaling between 2.5 and 8.11 m h−1, ranges that cover most of the absorption column operation conditions. Experimental data and correlations for the pressure drop, the liquid holdup and the gas–liquid mass transfer coefficients (kL) for modified Raschig ring packed columns are presented. The influence of the gas and the liquid velocities on the column hydrodynamics and the mass transfer coefficients have been obtained experimentally and also, have been compared with literature data.

Wall-to-Bed Mass Transfer in a Three-Phase Fluidized Bed with Twisted Tape as Internal

Experimental data on limiting current are obtained at point electrodes fixed flush with the inner surface of the outer cylinder of a three-phase fluidized bed electrochemical reactor in the presence of coaxially placed twisted tape inserts. Wall-to-bed mass transfer coefficient data are computed from the measured limiting current data. Glass balls of different diameters are employed as solid phase, nitrogen as gas phase and an electrolyte belonging to ferri–ferro redox system as liquid phase. Twisted tapes of different pitch values and width values are used as turbulent promoters. It is found that the use of twisted tape turbulent promoters in the three-phase fluidized beds yielded an augmentation of 25 %. Further it is observed that the mass transfer coefficient values are relatively uninfluenced by the longitudinal distance, and the liquid and gas velocities within the range of values considered for these variables. Further, the mass transfer coefficient is found to decrease with increase in tape pitch, tape width and particle diameter. A correlation is provided for the estimation of mass transfer coefficient using least squares regression analysis.

Artificial neural network model for turbulence promoter-assisted crossflow microfiltration of particulate suspensions

In this study, an artificial neural network (ANN) model for the turbulence promoter-assisted crossflow microfiltration (CFMF) process was successfully established, in which the inlet velocity, transmembrane pressure (TMP) and feed concentration were taken as inputs, and the flux improvement efficiency (FIE) by turbulence promoter was taken as output. Using the trained ANN model, the FIE can be predicted under CFMF operation conditions that are not included in the training database. It reveals that the FIE first increases and then decreases with increasing either TMP or inlet velocity, and increases with increasing feed concentration. Among three input variables, TMP has the most important effect on the FIE. The optimization of MF operation conditions was largely dependent on the feed concentration. The high FIE can be obtained by exerting both high inlet velocity (>0.7m/s) and low TMP (<30kPa) at a relatively low feed concentration (<1g/L), and both high inlet velocity (>0.7m/s) and high TMP (>70kPa) at a relatively high feed concentration (>8g/L). This study provides a useful guide for the applications of turbulence promoter in CFMF processes.

Direct observation of single- and two-phase flows in spacer filled membrane modules

Spacers are introduced in membrane modules as turbulence promoter to enhance mass transfer while the detailed hydrodynamics inside the flow channel is hard to recognize. The situation becomes more complicated when air bubbles are incorporated as second phase, i.e. air sparging processes. In this work, transparent modules with precise slit accommodating for spacer were fabricated to visualize the behavior of single- and two-phase flows. For single-phase flow, dye injection method and pressure drop measurement were conducted in channel with and without net-type spacer. The results show that with the incorporation of the spacer the critical Reynolds number decreases from 1000 to 100 approximately which is lower than channel with ladder-type spacer always adopted by researchers. For two-phase flow consisted of liquid and air bubbles, the behavior of bubbles in coalescence system (water–air) and non-coalescence system (ethanol–water–air) were investigated respectively. With stationary liquid in tubular module, the bubble size increases accordingly with the increase of the gas flow rate while the size of the bubbles is obviously shrunken in non-coalescence systems. In empty rectangular channel, the smallest bubble we found (2mm approximately) in non-coalescence system is larger than the channel height. Therefore bubbles of both systems could keep contact with channel wall which equips them with ability to clean fouling. Three different flow behaviors of two-phase flow are classified from the flow visualization in channel with diamond-like spacer D1. For both systems, working conditions in region (III) is optimal since the flow distribution is well managed. The threshold of region (III) is higher in non-coalescence system, because higher liquid and gas flow rate is required to sweep stagnant bubbles away. Higher porosity contributes less to the energy saving therefore porosity between 0.887 and 0.917 for the test spacer geometry is appropriate. Ladder-type spacers could avoid maldistribution of bubbles, which are suitable for conditions required lower gas and liquid flow rate.

Heat Transfer Enhancement Using Angled Grooves as Turbulence Promoters

An experimental study is conducted on a simulated internal cooling channel of a turbine airfoil using angled grooves and combination of grooves-ribs to enhance the heat transfer from the wall. The grooves are angled at 45 deg to the mainstream flow direction and combinations of four different geometries are studied that include (1) angled grooves with a pitch, p/δ = 10, (2) angled groove with a larger pitch, p/δ = 15, (3) combination of angled groove and 45 deg angled rib, and (4) combination of angled groove with transverse rib. Transient liquid crystal experiments are conducted for a Reynolds number range of 13,000–55,000, and local and averaged heat transfer coefficient values are presented for all the geometries. Pressure drops are measured between the inlet and the exit of the grooved channel and friction factors are calculated. The combination of the angled groove and 45 deg angled rib provided the highest performance factor of the four cases considered, and these values were higher or comparable to among the best-performing rib geometries (45 deg broken ribs) commonly used in gas turbine airfoils.

Experimental Investigation of Turbulence Promoters Effect on Characteristics of Linear Cascade Compressor

In this paper, considering the importance of inlet flow turbulence intensity in the combustion chamber, the effect of flow turbulence on characteristics of the flow wake within a linear compressor cascade has been experimentally investigated. To this purpose, two wires were installed along each blade an d their effects on average velocity, turbulence intensity at Re. Number 45500 were studied. One–dimensional hot wire anemometer was used to measure wake parameters. The results showed that turbulence promoters increased the maximum turbulence intensity in blades wake.

超臨界流体中に存在するスペーサまわりの伝熱流動に関する数値解析

As one of key issues on the thermal design of supercritical water reactors (SCWRs), it is required that the maximum fuel cladding surface temperature becomes less than 700℃. Then, the authors propose a heat transfer enhancement method by spacers. In addition, the spacer has a couple of small wing-shape ribs in order to increase the turbulent heat transfer. Preliminary analyses were performed to investigate numerically the heat transfer performance of a currently proposed ribbed-spacer under the supercritical water condition. Numerical parameters are the inlet velocity and heat flux. Moreover, as attack angles of the ribs to the spacer, three conditions were provided; 0°, 15° and 30°. From the predicted results, it was confirmed that the ribbed-spacer is effective as a turbulence promoter.

Enhancement of phase change along the long tubes of a latent heat storage unit

This paper presents the results of an experimental study on the solidification along a long horizontal tube submersed in liquid PCM and the enhancements due to the use of a turbulence promoter. The experimental rig consists of a long copper tube fixed coaxially inside a larger PVC tube filled with liquid PCM (water). The working fluid at a temperature lower than the PCM phase change temperature flows inside the copper tube provoking the solidification of PCM over its surface. The PVC tube is fitted with three windows along its length for observation and photographing the solidified PCM interface. The tests were realized for values of Reynolds number from 741 to 2280 and temperatures varying from -5 to - 20?C with and without turbulence promoter showed their effects on the interface position, the solidification velocity and how they were enhanced because of the turbulence promoter.

AN EXPERIMENTAL STUDY OF THE CONVECTIVE HEAT TRANSFER ENHANCEMENT: APPLICATION OF TURBULENCE PROMOTERS

This work presents an experimental study addressing the effects of turbulence promoters on heat transfer rate at circular cylinder in external cross flow. Within this framework, the work focuses on assessing the effects of three kind of turbulence promoters (with circular, square and hexagonal cross sections) on convective heat transfer enhancement. The distance from turbulence promoters to the circular cylinder (50, 100 and 150mm upstream), as well as the free stream velocity inside the wind tunnel (Reynolds number) were the parameters investigated. The validation of the experimental methodology was performed by comparing the present results with empiric correlations available in the literature. The main results indicate that the convective heat transfer coefficient was enhanced when using turbulence promoters. The highest heat transfer enhancement obtained was around 25% correponding to the case of square turbulence promoter placed closely (50mm) to the circular cylinder. Finally, it is worth mentioning that all the experimental results for the convective coefficient were condensed in a new empirical correlation with good accuracy.

Mass Transfer Enhancement Using Extensions as Turbulence Promoters

Mass Transfer Enhancement Using Extensions as Turbulence Promoters

Energetic consideration and flux characteristics of roughed-surface membrane in presence of reversing shear

Flux characteristics and energy dissipation for oscillatory flat surface membranes roughed with turbulence promoters (TP) have been investigated both theoretically and experimentally. The combined effect of oscillations and TP has proven effective in enhancing the microfiltration flux by an almost order of magnitude over that for a non-oscillatory flat surface membranes, and close to four times higher than that achieved in the absence of TP under the same oscillation conditions. The power dissipation was determined using the drag and inertia forces acting on individual promoters, as well from the solution of the boundary layer equations with time-invariant eddy viscosity for large and small roughness conditions. Using higher oscillation frequency and lower amplitudes was found to be more effective for flux enhancement and energy utilization where near self cleaning conditions were achieved at specific energy consumption of less than ∼0.5kWh/m3 filtrate. Both grooved and flat TP gave similar results in terms of flux enhancement and energy consumption per unit filtrate, with the former slightly better. Membrane fouling was found to follow the intermediate blocking fouling mechanism for oscillatory membranes as compared to cake filtration for non-oscillating systems. The quasi-steady state flux was satisfactorily predicted using an analytical model based on oscillatory rough surfaces with R2=0.95.

Computational fluid dynamics modeling and experimental study of continuous and pulsatile flow in flat sheet microfiltration membranes

Due to the significance of fluid flow hydrodynamics on the membrane surface, pulsatile flow was investigated as a functional approach to promote membrane performance. As a turbulence promoter, pulsatile flow in the membrane channel and its effects on the membrane performance were experimentally and numerically studied. A three-dimensional computational fluid dynamics (CFD) model was used to simulate cross-flow microfiltration process and fluid flow, including porous media by solving Navier-Stocks equations coupled with the Darcy's law. Two different boundary conditions were imposed at the module entrance: simple continuous fluid flow and pulsatile flow. Permeate flux, resistances, and shear forces were calculated in different operating conditions in which the experimental and simulated data showed a good agreement. It was found that shear force was the highest at the entrance region and then decayed along the flow direction for both continuous and pulsatile flow. Besides, two different types of pulsatile flow were considered; sinusoidal pulse flow and step function flow in which the latter led to higher shear forces and consequently higher permeate flux was obtained.

Effect of packing geometry on the rate of mass and heat transfer at a vertical tube imbedded in fixed bed under single and two phase flow

Rates of liquid–solid mass transfer and heat transfer (by analogy) were studied in an annular reactor with a packed annulus. Two types of inert fixed bed packing were used namely, cylinders and Raschig rings. The electrochemical technique which involves measuring the limiting current of the cathodic reduction of ferricyanide ion in a large excess of sodium hydroxide was used in the present study. Variables studied are packing geometry, packing size, gas and liquid superficial velocities and physical properties of the solution. The presence of inert fixed bed in the annulus enhanced the rate of mass transfer and the rate of heat transfer at the outer wall of the inner cylinder by a factor ranging from 1.1 to 6.1 depending on the packing geometry, particle size and both the liquid and gas superficial velocities. The present data were compared with the previous data on the packed annulus with inert spherical packing. For single phase liquid flow the mass transfer enhancement ratio increases in the order: Raschig rings>cylinders>spheres, while in the case of two phase flow, spheres gave the highest enhancement ratio. For the present range of conditions it was found that, as the particle size decreases the enhancement ratio increases. All data were correlated in the form of dimensionless equations.Possible practical applications of the present study such as design of fixed bed reactor internal cooler, prediction of the rate of diffusion controlled corrosion of vertical tube cooler imbedded in a fixed bed reactor and design of annular double tube catalytic and electrochemical reactors with a fixed bed turbulence promoter were highlighted.

Characterization of fluid dynamics in spacer-filled channels for membrane filtration using Doppler optical coherence tomography

The efficiency of membrane filtration is limited by the presence of concentration polarization (CP) and membrane fouling. The development of novel techniques to mitigate these negative effects requires an in-depth understanding of the fluid dynamics inside the fluid channel filled with a spacer that serves as a turbulence promoter. The current study was to apply Doppler optical coherence tomography (OCT) to characterize the velocity profile normal to the membrane surface, which is difficult to obtain by conventional techniques used for membrane characterization. The Doppler OCT system was used to visualize the flow patterns inside a unit cell of the spacer in a modified membrane filtration module. The orientation of the spacer was varied with respect to the bulk flow direction. A series of Doppler images was obtained to demonstrate the subtle interactions between the fluid and the spacer filaments; these characterization results then were used to interpret the performance variation of a reverse osmosis (RO) process with the same spacer configurations. The experimental results presented in this paper validate the ability of the Doppler OCT technique to characterize the fluid dynamics in the spacer-filled channel during membrane filtration. The proposed approach will be a useful tool to optimize the spacer design for better filtration performance.

Liquid cooling enhancement by means of magnetic fields

The goal of this work is to provide a preliminary evaluation of a new high-potential concept for liquid cooling: an electrically conductive fluid can flow through channels, while magnetic fields promote flow-wall impinging, causing an effect analogous to channel ribs (turbulence promoters), but in principle the effect is controllable. In order to evaluate the magneto-hydrodynamic effect, DNS has been used to investigate the influence of non-uniform magnetic fields on liquid metal flowing through a two-dimensional channel. The dimensionless numbers governing this flow field are the Reynolds (Re) and the Stuart (St) numbers. The latter provides the relative weight of the Lorentz forces with respect to the inertial forces. A liquid-lithium flow in laminar regime has been simulated both in absence of the magnetic field (Re = 2000, St = 0) and for increasing values of the Stuart number (Re = 2000, St equal to 750, 1000 and 2000), with the aim of putting into evidence the effects of magnetic forces on the magnitude of convective heat transfer. For a channel with a hydraulic diameter of 2 cm, such Stuart number range corresponds to magnetic field intensities varying between 1 and 2T, which are technically feasible by using either electromagnets or permanent magnets. Simulation results show that the local Nusselt number can be up to 2.8 times higher than that evaluated in the non-magnetic case.